The data transmission speed of videophones is limited by the requirement that the modems of such devices compress the video data so that it can be transmitted within the narrow band width available on telephone lines. In other words, if an increase in band width were possible, continuous motion could be displayed for higher rates of observed motion.
In order to achieve video data compression, a compression algorithm is embedded in a chip within the coder/decoder (codec) of the videophone and is used by the codec to compress the video data so that it may be sent over the narrow band width of a telephone line. Therefore, the better the compression algorithm, the higher the compression ratio and the better the transmission of video motion data. A typical videophone terminal includes a video camera or other imager and a display in combination with the codec for compressing the local video image prior to its transmission to a remote location and for expanding the remote video image after its reception from the remote location.
It is often necessary to test the operation of such videophones in the field. For this test, it is desirable to provide a controlled and repeatable optical excitation of the visual field as seen by the videophone. In the past, such testing has been performed by using a mechanical metronome as a visual source observed by a video camera for generating video test signals.
Although a metronome provides repeatable action, it produces only a minimal change in the visual field. If most of the observed camera field remains static, a minimal disturbance, such as generated by the swing motion of a metronome, may be transmitted as smooth action in instances where realistic motion results in a much deteriorated performance. Such testing with a metronome therefore does not adequately stress the operation of the entire system. Although a repairman might wave a sheet of paper or some other object within the visual field, such a method would produce unsystematic and uncalibrated optical excitation of the video system, instead of a controlled and repeatable optical excitation.
Other prior art systems have employed test video signals, such as the method and apparatus described in U.S. Pat. No. 5,313,280 to M. Straus, the entire contents of which are incorporated herein by reference. This patent describes the use of a video disk player for playing disks containing computer generated images, such as the image of a rotating color wheel, to provide video signals for testing the operation of a codec. However, such electrically generated signals bypass the originating camera and its associated circuitry, and therefore do not exercise the entire video system. In addition, the output signals of the video camera of a videophone are difficult to simulate because they often have proprietary video formats.
Other prior art systems employ video test patterns that are displayed on a video display placed in the visual field of the videophone to be tested. However, improper synchronization of the videophone and the scan phase of the video signal producing the pattern may introduce scan mismatch artifacts, thereby obscuring test results.
Finally, numerous prior art systems employ static test patterns or charts to test the operation of a video camera or display. However, static test patterns and charts are incapable of testing the motion compression algorithms of the codec and therefore are unsuitable for testing the entire video system.